Endothelial Expression of Class II MHC Proteins: A New Layer of Complexity in Kidney Transplantation
Authors: Matthew D. Griffin, Andrew F. Malone
Journal: Journal of the American Society of Nephrology (2023)

An understanding of the MHC genes and the proteins that they encode sits at the very core of our successes and failures in organ transplantation. Uniquely, MHC proteins represent the predominant source of genetically distinct (allo-)antigens toward which antidonor T-cell (cellular) and B-cell (humoral/antibody) immune responses are directed after transplantation as well as the primary antigen-presenting proteins responsible for initiating and propagating these responses. It is not surprising, therefore, that the extent to which the donor and recipient of a transplanted organ are matched or mismatched for all of their MHC-encoding genes is fundamentally linked to the risk of rejection and graft loss.1 Although the development of highly effective immunosuppressive drug regimens has blunted the impact of this association, the potential for discrepancies between donor and recipient MHC protein amino acid sequences to trigger adverse immune responses remains the Achilles heel of our strategies to achieve long-term, stable transplant function with a minimum of treatment-related toxicity.2 In humans, the MHC system constitutes two broad families of proteins—class I and class II HLA—which differ in their basic structures and expression patterns and in the T-cell subtypes to which they present peptide antigens. For kidney transplants, mismatched class II HLA (HLA II) proteins frequently serve as the target antigens for donor-specific antibodies (DSA), which are associated with high risk of antibody-mediated rejection and reduced graft survival.3 The three major subtypes of HLA II protein—DR, DQ, and DP—are each constitutively expressed by professional antigen-presenting cells of the immune system (dendritic cells, monocyte/macrophages, and B cells) but may also be expressed within the kidney by endothelial cells, epithelial cells, and fibroblasts under inflammatory conditions. HLA II proteins expressed on the surface of microvascular endothelial cells in transplanted kidneys serve as an important target for the harmful effects of DSA.4 While the three HLA II subtypes have varying degrees of genetic heterogeneity at population level, studies of cohorts of kidney transplant recipients have revealed that mismatching for immunogenic HLA-DQ epitopes may be more likely to stimulate harmful DSA than mismatches for HLA-DR.5–8 This suggests that there are mechanisms at play beyond mismatch number which influence the immunogenic potential of individual donor HLA II proteins. Elucidation of these mechanisms could, therefore, reveal new approaches to limiting the likelihood that adverse antidonor immune responses will emerge if and when the intensity of immunosuppressive therapy is lowered. In the current issue of JASN, Béland et al. provide an intriguing new insight into this area by demonstrating that HLA-DQ, DR, and DP proteins follow distinctly different expression dynamics on endothelial cells under inflammatory conditions.9 Using human endothelial colony forming cell (ECFC)–derived cell lines, the authors observed that there was lower surface expression and greater interindividual variability of expression of HLA-DQ compared with HLA-DR and HLA-DP proteins after stimulation with IFNγ. In pursuing this further, they assessed the relationship between HLA-DQ genotype and protein expression across their panel of ECFC-derived lines. This revealed that HLA-DQ protein expression varied according to genotype with higher surface expression observed for HLA-DQ5 and -DQ6 genotypes and lower surface expression observed for HLA-DQ7, -DQ8, and -DQ9 genotypes. To explore the clinical impact, the authors studied the relationship between specific HLA-DQ mismatches and microvascular inflammation scores in two cohorts of kidney transplant recipients, both analyzed in the presence and absence of HLA-DR mismatches. In the first cohort, they found a significant increase in microvascular inflammation scores (Banff ptc+g) among grafts with mismatches involving donor HLA-DQ5 and HLA-DQ6. This was confirmed for donor HLA-DQ5 mismatches in the second cohort. Given the accumulating evidence that HLA-DQ proteins and their immunogenic epitopes are important drivers of harmful antidonor immune responses,5–8 the results presented by Béland et al. are both interesting and timely. Although the study does not directly address the mechanisms underlying the observed differences in surface expression levels of class II HLA subtypes, the authors conclude, in their discussion, that a transcriptional explanation is unlikely. It is worth noting, however, that transcriptional variabilities among class II HLA alleles were reported over 20 years ago.10 More recently, Johansson et al. examined differences in mRNA expression among HLA genes and alleles using unique molecular identifiers, a quantifiable RNA sequencing approach.11 Regarding mRNA expression in heterozygous individuals, the highest degree of allelic imbalance was at HLA-C, -DPB1, -DQA1, and DQB1. This was in contrast to HLA-DRB1, for which there was very limited allelic imbalance of mRNA expression within individuals. The highest and lowest expressed HLA-DQB1 alleles were HLA-DQB1*05 and HLA-DQB1*06, respectively, while HLA-DQB1*02, -DQB1*04, and -DQB1*03 mRNAs were expressed at intermediate levels. In another study, Ferstl et al. showed allele-specific expression of DQB1 gene products during cytokine-driven maturation of monocyte-derived dendritic cells.12 In heterozygous individuals, differential expression of HLA-DQB1*0301 compared with DQB1*0501 and DQB1*0602 was observed. After stimulation, the ratio of HLA-DQB1*0301 to DQB1*0501 increased to 1.9-fold and HLA-DQB1*0301 to DQB1*0602 increased to 2.5-fold. This suggested dominance of HLA-DQB1*0301 expression over DQB1*0501 and DQB1*0602 and also indicated a variability among alleles in the transcriptional response to stimulation. Notably, these findings seem contradictory to those of Béland et al. who demonstrate a greater increase in HLA-DQ expression associated with the DQ1 genotype (HLA-DQB1*05:01, 05:02, 06:01, 06:02, 06:03, or 06:04) than with the DQ3 genotype (HLA-DQB1*03:01, 03:02, 03:03) and a dominant negative effect of HLA-DQ3 over HLA-DQ1. However, as Béland et al. measured protein rather than mRNA, the discrepancy may be explained by post-translational mechanisms which determine the final levels of HLA-DQ protein complexes on the cell surface. Cell type–specific differences in the expression of class II HLA proteins could also underlie such divergent observations. Indeed, the predominant use by Béland et al. of ECFC-derived cells as opposed to immortalized cell lines or peripheral blood mononuclear cells represents a strength of the study given that class II HLA proteins on the microvascular endothelium of kidney allografts are likely to be the most important target of DSA. Regardless of the molecular mechanisms driving allele-specific HLA-DQ antigen expression, the transplant community can benefit from studies that identify HLA-DQ alleles with increased potential to stimulate harmful antidonor immune responses. Specifically, the results of this novel and innovative study, if confirmed in vivo, may eventually help us to better navigate the complexities of HLA-DQ matching during organ allocation. Furthermore, the positive correlations observed between donor/recipient mismatches for "high expresser" HLA-DQ genotypes and the development of microvascular lesions hint at the potential to apply these insights to individualize the intensity of immunosuppressive therapy. To understand the true clinical relevance, detailed studies will be needed to explore whether specific donor HLA-DQ genotypes and donor/recipient mismatches are associated with increased expression of class II HLA on graft microvascular endothelium, higher levels of de novo DSA development, and higher risk for acute rejection and premature allograft loss.